Synthetic polymeric membranes are used for separation of species by dialysis, electrodialysis, ultrafiltration, cross flow filtration, reverse osmosis and other similar techniques. One such synthetic polymeric membrane is disclosed in Australian Patent Specification No. 505,494 of Unisearch Limited.
The memberane forming technique disclosed in the Unisearch Patent is broadly described as being the controlled uni-directional coagulation of the polymeric material from a solution which is coated onto a suitable inert surface. The first step in the process is the preparation of a "dope" by dissolution of a polymer. This is said to be achieved by cutting the hydrogen bonds (which link the molecular chains of the polymer together) with a solvent. After a period of maturation, the dope is then cast onto a glass plate and coagulated by immersion in a coagulation bath which is capable of diluting the solvent and annealing the depolymerized polymer which has been used. According to the one example given in this specification, the "dope" consisted of a polyamide dissolved in a solvent which comprised hydrochloric acid and ethanol.
In another membrane forming technique, the liquid material out of which the membrane is cast is a colloidal suspension which gives a surface pore density that is significantly increased over the surface pore density of prior membranes.
According to that technique, a thermoplastic material having both relatively non-crystalline and relatively crystalline portions is dissolved in a suitable solvent under conditions of temperature and time which cause the relatively non-crystalline portions of the thermoplastic material to dissolve whilst at least a portion of the relatively crystalline portion does not dissolve but forms a colloidal dispersion in the solvent. The colloidal dispersion and solvent (i.e. the "dope") is then coated onto a surface as a film and thereafter precipitation of the dissolved thermoplatic portion is effected to form a porous membrane.
Such aliphatic polyamide membranes suffer from disadvantages which limit their commercial usefulness and applicability. For example they exhibit dimensional instability when drying and may shrink by up to 7%. Thus, it is essential that they be kept moist prior to and after use. Furthermore, it has not been possible to generate chemical derivatives of the membrane matrix which restricts the situations to which the membrane may be applied.
Another disadvantage is that such polyamide membranes are fundamentally unstable and eventually become brittle on storage. The instability has been carefully investigated by I. R. Susantor of the Faculty of Science, Universitas Andalas, Padang, Indonesia with his colleague Bjulia. Their investigations were reported at the "Second A.S.E.A.N. Food Waste Project Conference", Bangkok, Thailand (1982) and included the following comments regarding brittleness:
"To anneal a membrane, the thus prepared membrane (according to Australian Pat. No. 505,494 using Nylon 6 yarn) is immersed in water at a given temperature, known as the annealing temperature, T in degrees Kelvin. It is allowed to stay in the water a certain length of time, called the annealing time. For a given annealing temperature, there is a maximum annealing time, t(b) in minutes, beyond which further annealing makes the membrane brittle. Plotting ln 1/t(b) versus 1/T gives a straight line. From the slope of this line it can be concluded that becoming brittle on prolonged annealing is a process requiring an activation energy of approximately 10.4 kilocalories/mole. From the magnitude of this activation energy, which is of the order of van der Waals forces, the various polymer fragments are probably held together by rather strong van der Waals forces or hydrogen bond(s)."
We have confirmed that the brittleness is due to a recrystallization of water-solvated amorphous polyamide. In some cases (such as polyamide 6) brittleness occurs within 48 hours of immersion in distilled water (pH7) at 80.degree. C. Colorimetric --NH.sub.2 end group analysis has shown that there is no significant hydrolysis of the amide groups during this time. As would be expected, the rate of embrittlement is catalysed by dilute acids (eg: pH of 1.0) due to nitrogen protonation and subsequent solvation. This effect explains the apparently low acid resistance of the polyamide membranes. However colorimetric determination of both --NH.sub.2 end groups and --COOH end groups has shown that the effect is due to crystallization rather than acid catalysed hydrolysis.
That most of the brittleness is due to physical effects rather than chemical decomposition or chemical solvation (at least for dilute acids) is shown by the extreme embrittlement caused on standing 5 minutes in absolute ethanol.
The problem of crystallization of the aliphatic polyamide material can be overcome by cross-linking portions of the polyamide through the reaction of a bis-aldehyde with the membrane matrix as is described in our International Patent Application No. PCT/AU84/00015 "Cross-Linked Porous Membranes". However, the chemical derivatives of such cross-linked polyamide membranes are limited to those which can be prepared in water and thus those membranes can not be used to provide derivatives which do not lend themselves to aqueous synthesis such as the ester of 4-hydroxybenzaldehyde. Furthermore, the density of derivatives prepared in water may not be as large as desired. For example, the up-take of resorcinol in the glutaraldehyde cross-linked membrane of example 2 of our above mentioned International Patent Application was only 0.2% of the dry weight of the membrane.
It is an object of this invention to provide aliphatic polyamide porous membranes which lend themselves to the preparation of chemical derivatives which are not readily available by aqueous synthesis and to increased density of derivatives which otherwise may be prepared in water.